On the Impact of Neutron Star Binaries Natal-Kick Distribution on the Galactic r-process Enrichment

TL;DR

This study investigates how the distribution of natal kicks in neutron star binaries affects the enrichment of galaxies with r-process elements, highlighting the importance of natal kicks and merger timescales in galactic chemical evolution.

Contribution

It provides a model linking neutron star binary natal kick distributions with their contribution to galactic r-process enrichment, incorporating galaxy growth and star formation history.

Findings

Up to 40% of neutron star binaries do not contribute to r-process enrichment by present day.

The results are insensitive to the specific delay-time distribution function used.

Assuming a constant coalescence timescale of 100 Myr approximates the impact of the detailed delay-time distribution.

Abstract

We study the impact of the neutron star binaries' (NSBs) natal kick distribution on the Galactic r-process enrichment. We model the growth of a Milky Way type halo based on N-body simulation results and its star formation history based on multi epoch abundance matching techniques. We consider the NSBs that merge well beyond the galaxy's effective radius ($>2\times R_\mathrm{eff}$) do not contribute to Galactic r-process enrichment. Assuming a power-law delay-time distribution (DTD) function ($\propto t^{-1}$) with $t_\mathrm{min}=30$ Myr for binaries' coalescence timescales, and an exponential profile for their natal kick distribution with an average value of 180 km s$^{-1}$, we show that up to $\sim$ 40% of all formed NSBs do not contribute to r-process enrichment by $z=0$, either because they merge far from the galaxy at a given redshift (up to $\sim$ 25%) or have not yet merged by today ($\sim$15%). Our result is largely insensitive to the details of the DTD function. Assuming a constant coalescence timescale of 100 Myr well approximates the adopted DTD with 30% of the NSBs not contributing to r-process enrichment. Our results, although rather dependent on the adopted natal kick distribution, represent a first step towards estimating the impact of natal kicks and DTD functions on r-process enrichment of galaxies that would need to be incorporated in the hydrodynamical simulations.